Unfortunately it can't be totally accurate going back that far. The switch over from Julian Calendar to Gregorian Calendar took about 350 years. Going back that far to see what day of the week something happened centuries ago won't be correct.
It will be correct according to Gregorian standards. You could use a Julian calendar for the modern times if you wanted to. Dates might not line up with what everyone else uses, but it would technically be correct according to the calendar you're using. Really no different than kilograms vs pounds, Celsius vs Fahrenheit, etc.
Anyways, it's not like Apple developers had to write code for each year. The algorithm is relatively simple, so calendars for any year can be generated on the fly.
i kept scrolling up and the calendar stops at 4716 BC. Kind of a weird year to pick. If it is an algorithm it should go back indefinitely.
If I had to guess, the limit is likely due to the length of an int or something. Similar to the
Year 2038 problem just in the other direction. Or 4, 7, 1 and 6 were some developer's lucky numbers in the fortune cookie they got when they went out to lunch.